Assuming your theory is true, then fluoridation for only a short period of time provides lifelong benefits. So, why would it be necessary to fluoridate water forever? Also, it doesn't explain my great-grandmother. :-)
You mostly need flouride when you are younger. That’s why you get flouride treatments as a kid between age 6 and 12, and not as an adult.
Its necessary to floruidate water forever because there are always new kids getting born. Its not like it changes genetics and makes kids be born with super teeth.